Homo- and heteroanionic alkali metalaza-enolate aggregates derived from o-methylvalerolactim ether

Abstract

The reaction of o-valerolactim ether with BuM (M = Li, Na, K) in the presence of the Lewis donors thf, tmeda, pyridine and pmdta resulted in the crystallisation and structural characterisation of a series of homo- and heteroanionic aggregates, which are also either homo or heterobimetallic. All the structures incorporate an aza-enolate (1-aza-allyl) anion (=R) derived from deprotonation of the lactim ether at the α-C, and its subsequent electronic rearrangement such that the metal bonds to the amido N. Lithiation resulted in crystals of [R₃{R(Me₂SiO)}Li₄·(thf)₄], 1, with Me₂SiO⁻ incorporated from the base induced decomposition of silicone grease; the linked dimer [R₄Li₄·(tmeda)₃], 2, which has both terminal and bridging tmeda molecules; the octanuclear cluster [R₆–μ⁶-O–Li₈·(pyr)₂], 3, which is constructed around an O²⁻ anion; and [R(CH₂CHO)₂Li₃·pmdta]₂, 4, which incorporates lithium ethenolate, the base induced decomposition product of thf. Sodiation resulted in crystals of the simple dimeric complex [RNa·tmeda]₂, 5, while potassiation resulted in the heterobimetallic cluster [R(MeO)KLi·(tmeda)]₄, 6, incorporating a MeO⁻ anion due to its nucleophilic displacement from the valerolactim ether as MeOK by ⁿBu and subsequent anion exchange with ^tBuOLi. Solution NMR studies on the reaction between RH and ^tBuLi in the absence of a Lewis donor revealed that both the rate of deprotonation at the α-carbon and nucleophilic substitution of the ether functionality decrease substantially. The cyclic 2-^tBu-imine, as the thermodynamic product, becomes the dominant product, indicating that a Lewis donor solvent is necessary for efficient deprotonation to occur.